1. Field of the Invention
The present invention relates in general to an apparatus and method for detecting a cell in a mobile communication system and, more particularly, to an apparatus and method for detecting a cell containing a base station to be synchronized with a mobile device in an LTE (Long Term Evolution) system.
2. Description of the Related Art
A mobile communication system based on an OFDM (Orthogonal Frequency Division Multiplexing) system such as an LTE system is normally composed of a base station and a mobile device (also referred to as a User Equipment (UE), a mobile terminal or a mobile station). A base station and a mobile device establish a link for data transmission. A link from a mobile device to a base station is referred to as an uplink, whereas a link from a base station to a mobile device is referred to as a downlink.
A signal in the downlink is transmitted in the form of continuous frames each having 10 msec size. A 10 msec frame is composed of two 5 msec half frames. In addition, a single half frame is composed of five 1 msec sub frames. Each sub frame has a plurality of slots and symbol units. In the OFDM system, a symbol is used as a frequency conversion unit.
Generally, a mobile device ascertains a point of time to synchronize with a neighboring base station before performing FFT (Fast Fourier Transform). For this, a mobile device detects the primary synchronization channel (PSCH) symbol containing information about boundaries of a half frame in a signal received from a neighboring base station through the downlink. After finishing the detection of the PSCH symbol, a mobile device detects the secondary synchronization channel (SSCH) symbol containing information about a base station on the frequency axis. Through the PSCH symbol and the SSCH symbol, a mobile device can obtain information about all frame boundaries and information about a base station. Then by using such information, a mobile device can detect a neighboring cell.
Namely, the acquisition of synchronization between a base station and a mobile device begins with detecting boundaries of a half frame by using the PSCH symbol. Since there is no timing information, the detection of the PSCH symbol is performed immediately on the time axis. Once the detection of the PSCH symbol is finished, a mobile device can find only boundaries of a half frame. Since the PSCH symbol is always transmitted in the same position from a base station, and since such positions are unchanged for each half frame, a mobile device cannot ascertain boundaries of the entire frame. However, if the SSCH symbol is detected, a mobile device can find all frame boundaries. In other words, since the configuration mode of the SSCH symbol (i.e., m0, m1 sequence mode) is distinguished by a half frame in a single frame, it is possible to find boundaries of the entire frame according to the position of the SSCH symbol.
Now, a process of detecting a synchronization channel will be described in detail with reference to FIG. 1, which is a flow diagram illustrating a conventional method for detecting a synchronization channel.
Referring to FIG. 1, a mobile device receives a signal transmitted from a base station that controls neighboring cells around that device in step 110. Then the mobile device detects the PSCH symbol in the received signal in step 120. Through the detected PSCH symbol, the mobile device roughly finds the boundary of a downlink half frame. Next, the mobile device determines whether the detection of the PSCH symbol is finished in step 130. Here, the mobile device temporarily stores the received signal in a memory until the PSCH symbol detection is finished.
After the PSCH symbol detection, the mobile device determines whether the detected PSCH symbol is valid in step 140. If the PSCH symbol is valid, the mobile device detects the SSCH symbol at an estimated position in step 150. Namely, when the PSCH symbol is completely detected, the mobile device estimates the position of the SSCH symbol by using the detected PSCH symbol. Specifically, the mobile device finds the boundary of a half frame in the PSCH symbol and estimates the position of the SSCH symbol. Then the mobile device detects the SSCH symbol in the signal temporarily stored in the memory. The SSCH symbol contains information about all frame boundaries and information about a base station. Here, “base station information” means the base station ID used for identifying a base station.
When the SSCH symbol is also detected, the mobile device then can ascertain information all frame boundaries and information about a base station in step 160. Then the mobile device can detect a cell controlled by the base station corresponding to ascertained information.
In order to detect the SSCH symbol, the mobile device should convert a symbol estimated to be the SSCH symbol into that on the frequency domain. The position of the estimated SSCH symbol may be found after the boundary of a half frame is detected. That is, the SSCH symbol may be detected only after the PSCH symbol is detected. However, since the detection of the SSCH symbol is delayed until the detection of the PSCH is finished, it may take a longer time to completely detect information about a base station and then to perform synchronization.
In addition, the mobile device should store all signals received while the PSCH symbol is detected. Namely, since the SSCH symbol is not known to be located at which region of a received signal, the mobile device must store all signals during the detection of the PSCH symbol.